Dry-process nonwoven pulp fabric composed of united layer structures

ABSTRACT

Disclosed is a dry-process nonwoven pulp fabric composed of united layer structures, which contains heat-bondable synthetic fibers and comprises surface layer portions on both sides in which the synthetic fibers are heat bonded to one another and which has a basis weight of more than 5 g/m 2  to 12 g/m 2 , and an internal layer portion in which heat-bondable synthetic fibers and pulp fibers are mixed at a ratio of 20/80 to 60/40% by weight and the synthetic fibers and/or the synthetic fibers and pulp fibers are heat bonded to one another and which has a basis weight of 8 to 240 g/m 2 , wherein the front and back surface layers and the internal layer portion are united as a whole by heat bonding of the synthetic fibers to one another, the ratio of a strength in a lengthwise direction to that in a crosswise direction is from 0.8 to 1.2 in both dry and wet states, the ratio of a strength in a dry state to that in a wet state is from 0.6 to 1.1, the water absorption is from 8 to 20 g/g, and the total basis weight is from 20 to 250 g/m 2 . The resulting dry-process nonwoven pulp fabric is free from the shedding of fibers, sufficient in water absorption properties and high in wet strength, and therefore suitable for wipers, kitchen sheets and the like used in a wet state.

TECHNICAL FIELD

The present invention relates to a dry-process nonwoven pulp fabricwhich is strong even when it is wetted. More particularly, the presentinvention relates to a nonwoven fabric used in a wet state, which issuitable for wipers for material use such as floor use, kitchen use orautomobile use, for kitchen sheets for cleaning of cooking utensils,wiping of foodstuffs, drip absorption and the like, or for wipers forhuman use such as face use, hand use or body use. Further, in thepresent invention, even when the nonwoven fabric is used in a dry state,water exists in these uses in many cases, so that similar effects can beexpected. Furthermore, the present invention relates to a nonwovenfabric similarly suitable for hygienic materials such as sanitary goodsand diapers, or for medical use.

BACKGROUND ART

In a dry-process nonwoven pulp fabric which has hitherto been known, apulp fiber layer is used as a base, and a chemical binder resin such asa polyacrylic ester-based or polyvinyl acetate-based resin is sprayed orapplied on a surface layer thereof or impregnated throughout the wholethereof to form interfiber bonds.

In such a nonwoven fabric, when the binder amount is increased, theresin forms a film to become hard, which causes a reduction in waterabsorption ability. On the other hand, when it is decreased, not onlyshedding of pulp fibers increases, but also a decrease in strength,particularly in wet strength, is unavoidable.

Further, a method of imparting a powdery binder resin is conceivable. Inthis case, however, the number of adhesion points between the pulpfibers and the binder decreases to cause a tendency to increase sheddingfibers, although the film is difficult to be formed. It is thereforedifficult to produce a dry-process nonwoven pulp fabric excellent in wetstrength while securing moderate water absorption properties, decreasedshedding of fibers, strength available for practical use and the like.

As a dry-process nonwoven pulp fabric used for absorption insertmaterial use of sanitary goods or for domestic or industrial washinguse, there has been proposed a dry-process nonwoven pulp fabric in whichthe basis weight (METSUKE) of a surface layer composed of only a binderfiber is from 1 to 5 g/m², and the amount of the binder fiber containedin an internal layer portion is from 2 to 10% by weight (PublishedJapanese Translation of PCT Patent Application 2000-504792).

The strength of this nonwoven fabric is sufficient with respect toordinary strength for hygienic good use such as absorption insertmaterial use of sanitary goods. However, it is not mentioned at allwhether the nonwoven fabric is “strong even when wetted” or not, whichis an important point of the present invention. The nonwoven fabrichaving a technical content within the scope of this published patent isas small as 2 to 10% by weight in the amount of the binder fibercontained in the internal layer portion, so that it can not be said tobe strong even when wetted. This brings about a significant decrease instrength in a wet state to be unsuitable for wiping, and therefore iscompletely different from the intension of the present invention.

Further, there has also been proposed an air-laid nonwoven fabric havinga specified tearing strength, which is formed by heat bondingintersecting points of binder fibers (JP-A-2000-345454). However, wetstrength in compounding with pulp and water absorption properties arenot mentioned at all, and there is utterly no description which suggeststhe technique as a layer structure of the present invention.

Conventional nonwoven fabrics have the problems as described above.

DISCLOSURE OF THE INVENTION

The present inventors have made intensive studies in view of the presentstate as described above. As a result, the present inventors havediscovered that a dry-process nonwoven fabric which is free from theshedding of fibers, sufficient in water absorption properties andstrong, particularly sufficient in wet strength, can be obtained bymaking proper the basis weight (METSUKE) of heat-bondable syntheticfiber-containing front and back surface layers and increasing the amountof the heat-bondable synthetic fibers contained in an internal layer,thus completing the present invention. Best Mode for Carrying Out theInvention The present invention relates to a dry-process nonwoven pulpfabric (hereinafter also briefly referred to as a “dry-process nonwovenfabric”) composing united layer structures, which contains heat-bondablesynthetic fibers and comprises surface layer portions on both sides inwhich the synthetic fibers are heat bonded to one another and which hasa basis weight of more than 5 g/m² to 12 g/m², and an internal layerportion in which heat-bondable synthetic fibers and pulp fibers aremixed at a ratio of 20/80 to 60/40% by weight and the synthetic fibersand/or the synthetic fibers and pulp fibers are heat bonded to oneanother and which has a basis weight of 8 to 240 g/m², wherein the frontand back surface layers and the internal layer portion are united as awhole by heat bonding of the synthetic fibers to one another, the ratioof a strength in a lengthwise direction to that in a crosswise directionis from 0.8 to 1.2 in both dry and wet states, the ratio of a strengthin a dry state to that in a wet state is from 0.6 to 1.1, the waterabsorption is from 8 to 20 g/g, and the total basis weight is from 20 to250 g/m².

Embodiments of the present invention are illustrated below.

The nonwoven fabric of the present invention composed of a three-layerstructure comprising an internal layer containing heat-bondablesynthetic fibers and pulp fibers and surface layers containingheat-bondable synthetic fibers, between which the internal layer issandwiched, and is united by heat bonding.

In the present invention, the heat-bondable synthetic fibers which are amain component forming the surface layers or mixed with pulp may be any,as long as they are melted by heat and bonded to one another, and thepulp is fixed by a network structure caused by these interfiber bonds.Accordingly, the fibers using a polymer having high affinity with thepulp fibers are particularly preferred. Examples thereof include apolyolefin, a polyolefin grafted with an unsaturated carboxylic acid, apolyester, polyvinyl alcohol and the like.

Of these, as the polyolefinic heat-bondable synthetic fibers, there aresuitable sheath-core type or eccentric side-by-side type conjugatefibers. The polyolefins constituting the sheath or a peripheral portionof the fiber include polyethylene and polypropylene. The polymersconstituting the core or an internal portion of the fiber are preferablypolymers having a melting point higher than that of the sheath andunchangeable at a heat bonding treatment temperature. Such combinationsinclude, for example, polyethylene/polypropylene,polyethylene/polyester, polypropylene/polyester and the like. Thesepolymers may be modified within the range that the operation and effectof the present invention are not impaired. Further, they may befibrillated fibers. Examples thereof include SWP supplied from MitsuiChemicals, Inc.

When the heat-bondable synthetic fibers are fine in size, the number ofconstituent fibers increases. Accordingly, shedding fibers decrease, andthe hand feeling also becomes soft. When they are heavy in size, spacesamong fibers become large to form a bulky nonwoven fabric, and thescraping effect is also expected. Accordingly, the size of the fibersmay be selected depending on their use. However, the fineness thereof ispreferably from 0.5 dt to 50 dt, and more preferably from 0.8 dt to 30dt. Exceeding 50 dt unfavorably results in uncontrollable shedding ofthe pulp. On the other hand, less than 0.5 dt is unpractical because ofpoor nonwoven fabric productivity.

Further, the length of the heat-bondable synthetic fibers is preferablyfrom 1 to 15 mm. The shorter fibers result in better mixing propertieswith pulp, which causes a tendency to form the more uniform nonwovenfabric. However, less than 1 mm results in an approach to a powderystate, so that it is difficult to form the network structure caused bythe interfiber bonds. Accordingly, not only shedding of the pulp becomesuncontrollable, but also the strength as the nonwoven fabric decreases.This is therefore unfavorable because of lack of practicability. On theother hand, longer than 15 mm results in an increase in the strength ofthe nonwoven fabric, but fibers become liable to be entangled with oneanother during pneumatic transportation of the fibers in the productionof the nonwoven fabric, which unfavorably increases fiber agglomeratedefects. Particularly preferred is from 3 to 10 mm.

The surface layer may contain other fibers such as regenerated fiberssuch as rayon, semi-synthetic fibers such as acetate, synthetic fiberssuch as polyester, polypropylene, polyamide and Vinylon, and naturalfibers such as pulp, cotton and hemp, as well as the above-mentionedheat-bondable synthetic fibers. In this case, the ratio of theheat-bondable synthetic fibers in the surface layer is preferably from70 to 100% by weight, and more preferably from 85 to 100% by weight. Inthe case of less than 70% by weight, the possibility that shedding ofthe above-mentioned other fibers occurs increases, and not only theeffect of inhibiting shedding of the pulp in the internal layer portionis reduced, but also the wet strength decreases, which causes apractical problem.

These fibers forming the surface layer are heat bonded, and the pulp isfixed by the network structure caused by these interfiber bonds. Thebasis weight must be more than 5 g/m² to 12 g/m². Less than 5 g/m²results in the decreased amount of synthetic fibers having waterresistance and the decreased number of interfiber bonds. Accordingly,not only sufficient wet strength sustainable for wiping can not besecured, but also an increase in shedding fibers is liable to be broughtabout. Different from an application in which importance is attached toabsorptivity, such as hygienic materials, such a nonwoven fabric isunpractical to use for wipers, kitchen sheets and the like, because ofits easy fiber shedding when used for wiping. On the other hand,exceeding 12 g/m² results in too thick a heat-bonded layer having waterresistance, leading to insufficient water absorption to the internallayer and hard hand feeling. This is also unpractical. An increase inbasis weight results in an increase in strength, but in a reduction inwater absorption properties. However, the nonwoven fabric havingsufficient water absorption properties, sufficient strength even in awet state, no shedding of fibers and soft hand feeling can be obtainedwithin the range of the present invention.

Further, the synthetic fibers are contained in large amounts, so thatoil is well absorbed and compatibility with oil is good. Accordingly,there is also the advantage that stains are easily removed.

In the internal layer portion, the heat-bondable synthetic fibers andthe pulp fibers are united by heat bonding. The heat-bondable syntheticfibers may be the same as or different from those used in the surfacelayer. As the pulp fibers, pulverized pulp having a length of 0.2 mm to5 mm is preferred.

As for the mixing ratio of the heat-bondable synthetic fibers and thepulp fibers in the internal layer portion, when the ratio of theheat-bondable synthetic fibers is less than 20% by weight based on thetotal of the heat-bondable synthetic fibers and the pulp fibers,shedding fibers increase and strength, particularly strength in a wetstate, decreases. Such a nonwoven fabric can be used in an applicationin which importance is attached to water absorption properties andsoftness, such as a hygienic material, even when low in strength.However, it is unsustainable for use as a nonwoven fabric for wipingwhich is an object of the present invention. On the other hand,exceeding 60% by weight results in a decrease in the amount of the pulpfibers contributing to absorptivity, which causes not only lack of waterwiping properties and absorptivity, but also hard hand feeling. This istherefore unpractical.

An increase in the ratio of the heat-bondable synthetic fibers resultsin an increase in strength, but in a reduction in water absorptionproperties.

It has been therefore difficult for sufficient strength, particularlystrength in a wet state, to be compatible with water absorptionproperties.

The compatibility of these is very important for applications such aswipers and kitchen sheets. Within the ratio range of the presentinvention, the nonwoven fabric having sufficient water absorptionproperties and sufficient strength even in a wet state is firstobtained. Further, such a nonwoven fabric is small in the amount ofshedding fibers, and has such a sufficient strength that it can bewashed, squeezed and dried even when soiled, and can be repeatedly usedseveral times. Furthermore, the ratio of the heat synthetic fibers ishigh, so that the volume is small, and a reduction in thickness ispossible. Accordingly, the nonwoven fabric is excellent in handlingproperties, and also useful for space saving.

In the internal layer, the heat-bondable synthetic fibers and/or theheat-bondable synthetic fibers and pulp fibers are heat bonded to oneanother. The basis weight of this internal layer is preferably from 8 to240 g/m². Less than 8 g/m² results in too small the amount of the pulp,which causes insufficient water absorption properties. On the otherhand, exceeding 240 g/m² is unsuitable for this application because thewhole presents a plate form.

Further, as the whole nonwoven fabric, the front and back surface layersand the internal layer portion are also united by heat bonding theheat-bondable synthetic fibers to one another. Furthermore, it hassufficient heat sealing properties.

Such a nonwoven fabric may be produced by any method, as long as it is adry-process nonwoven fabric. However, a nonwoven fabric produced by anair-laid method is preferred. The nonwoven fabric produced by theair-laid method is preferred, because fibers forming the nonwoven fabricare randomly three-dimensionally orientated in a longitudinal, width andthickness directions of the nonwoven fabric.

Here, the nonwoven fabric produced by the air-laid method can beobtained in the following manner.

Fibers mainly composed of a specified amount of opened heat-bondablesynthetic fibers are conveyed while uniformly dispersing them in an airstream, and the fibers blasted out from a screen having fine pores whichis provided on a discharge portion are dropped on a metal or plastic netdisposed in a lower portion to accumulate the fibers on the net whilesucking air at an underside of the net. Then, a mixture of theheat-bondable synthetic fibers and the pulp fibers is similarlyaccumulated on the above-mentioned accumulated sheet. Fibers mainlycomposed of the heat-bondable synthetic fibers are further accumulatedon the resulting sheet.

Then, the whole is heat treated at a temperature at which theheat-bondable synthetic fibers sufficiently exhibit their bondingeffect, thereby being able to obtain the dry-process nonwoven pulpfabric of the present invention. In order to allow the fibers tosufficiently exhibit the bonding effect, heat treatment at a temperature15 to 40° C. higher than the melting point of a bonding component of theheat-bondable synthetic fibers is necessary.

In the nonwoven fabric thus produced by the air-laid method, the fiberscan be randomly three-dimensionally orientated in a flow, width andthickness directions of the nonwoven fabric. These are heat bonded, sothat no delamination occurs. Further, the nonwoven fabric produced bythe air-laid method is good in uniformity, so that variations inperformance are reduced.

Calender treatment or embossing treatment can also be further conductedas needed.

Further, in the nonwoven fabric of the present invention, the ratio of astrength in a lengthwise direction to that in a crosswise direction mustbe from 0.8 to 1.2, and preferably from 0.85 to 1.2, in both dry and wetstates. When either one of the strengths is low, a practical trouble isliable to occur.

Furthermore, the ratio of a strength in a dry state to that in a wetstate must be from 0.6 to 1.1, and preferably from 0.7 to 1.1. Less than0.6 results in a nonwoven fabric which is largely decreased in strengthin a wet state compared to that in a dry state, that is to say, weakenedwhen wetted. This departs from the intention of the present invention,and a practical trouble occurs. Further, in a wet state, the strengthincreases by surface tension among the fibers due to the presence ofwater, and exceeds 1 in some cases. This is within the scope of thepresent invention. However, the strength usually does not exceed 1.1,unless another means of some kind which bonds in the presence of waterexists.

It is necessary that a wiper, a kitchen sheet, a hygienic material andthe like to which the nonwoven fabric of the present invention isapplied have a moderate water absorption, and the water absorption mustbe from 8 to 20 g/g, and preferably from 10 to 18 g/g. Less than 8 g/gresults in lack of wiping properties and water retention in practicaluse, which causes a practical problem. On the other hand, exceeding 20g/g results in too much water content retained by the nonwoven fabricitself, which causes a problem with regard to its handling properties.

The basis weight of the whole nonwoven fabric is preferably from 20 to250 g/m², and can be suitably selected depending on its use.

The nonwoven fabric of the present invention may be united with anothersheet, as long as the intension of the present invention is notimpaired. For example, when it is united with a breathable sheet, thebreathable sheet is previously placed on a metal gauze in preparing thenonwoven fabric of the present invention, and the fibers are accumulatedthereon, thereby being able to easily form a composite sheet.

Sheets to be united include a dry-process nonwoven fabric, a wet-processnonwoven fabric, paper, a spun-bond nonwoven fabric, a melt-blownnonwoven fabric, a plastic net, a perforated film, a split yarn cloth, arough woven and knitted fabrics, a victoria lawn and the like. However,a sheet composed of a heat-bondable material is preferred, and onehaving high air permeability is preferred.

In the present invention, it has become possible to inhibit the sheddingof fibers and to increase the strength of the nonwoven fabric to such adegree as to be sustainable for wiping use or repeated use withoutimpairing water absorption properties by making proper the basis weightof the front and back surface layers containing the heat-bondablesynthetic fiber and increasing the amount of the heat-bondable syntheticfibers contained in the internal layer. Further, the nonwoven fabrichaving good wiping properties for water and oil, having no delaminationand very suitable for wiping use has been able to be obtained.Furthermore, the nonwoven fabric of the present invention has sufficientheat sealing properties, and is similarly suitable for hygienicmaterials such as sanitary goods or diapers, or for medical use.

In addition, the nonwoven fabric of the present invention is free from afear of a residual monomer due to the chemical binder resin, so that itis sanitary.

Moreover, no waste water is produced in the production process of thenonwoven fabric, and no monomer of the chemical binder is contained inexhaust, resulting in reduced environmental load.

EXAMPLES

The present invention will be illustrated with reference to thefollowing examples, but the invention should not be construed as beinglimited thereto.

Example 1

Sheath-core type conjugate fibers (F6, manufactured by Teijin FibersLtd., 2.2 dt, length: 5 mm) comprising polyethylene terephthalate as acore and polyethylene as a sheath were used as heat-bondable fibers forfront and back surface layer portions, and a mixture of 30% by weight ofconjugate fibers (Intack, manufactured by Chisso Polypro Fiber Co.,Ltd., 1.7 dt, length: 5 mm) comprising polypropylene as a core andcopolymer polyethylene as a sheath and 70% by weight of pulp (NB416Kraft manufactured by Weyerhauser Co.) was used as an internal layerportion to produce a nonwoven fabric at a heating temperature of 145° C.by an air-laid method.

The basis weight of the front and back surface layers was 6 g/m², andthe basis of the internal layer was 33 g/m². The thickness was 0.54 mm.

Physical properties such as strengths in a lengthwise direction and acrosswise direction, the ratio thereof, the ratio of a strength in a drystate to that in a wet state and water absorption are shown in Table 1.

The strength and water absorption were measured by the followingmethods:

(1) Strength: The strength was measured according to JIS L-1913, whereinthe width of a test piece was taken as 25 mm, and the clamp distance wastaken as 100 mm.

(2) Water Absorption: A test piece of 100 mm×100 mm previously weighedis immersed in water at 20° C. for 1 minute, and then paced on a glassplate inclined at an angle of 45 degrees for 1 minute. Then, the testpiece is reweighed, and the water absorption is calculated by thefollowing equations:

{circle around (1)} Water absorption per area (g/m²): The weight ofwater absorbed is divided by the area of the test piece, and indicatedin g/m².

{circle around (2)} Water absorption per its own weight (g/g): Theweight of water absorbed is divided by the weight of the test piece, andindicated in g/g.

Example 2

A nonwoven fabric was produced by the air-laid method in the same manneras with Example 1 with the exception that a mixture of the bondableconjugate fibers and pulp at a ratio of 90/10 was used in the front andback surface layer portions, the basis weight thereof was changed to 8g/m², the ratio of the bondable conjugate fibers to the pulp in theinternal layer was changed to 25/75, and the basis weight thereof waschanged to 49 g/m².

The thickness was 0.68 mm.

Example 3

A nonwoven fabric was produced by the air-laid method in the same manneras with Example 1 with the exception that the basis weight of the frontand back surface layer portions was changed to 10 g/m², the ratio of thebondable conjugate fibers to the pulp in the internal layer was changedto 40/60, and the basis weight thereof was changed to 100 g/m².

Comparative Example 1

A nonwoven fabric was produced by the air-laid method in the same manneras with Example 1 with the exception that the basis weight of the frontand back surface layer portions was changed to 4 g/m², the ratio of thebondable conjugate fibers to the pulp in the internal layer was changedto 8/92, and the basis weight thereof was changed to 52 g/m².

Comparative Example 2

A nonwoven fabric was produced by the air-laid method in the same manneras with Example 1 with the exception that the basis weight of the frontand back surface layer portions was changed to 16 g/m², the ratio of thebondable conjugate fibers to the pulp in the internal layer was changedto 70/30, and the basis weight thereof was changed to 23 g/m².

Comparative Example 3

An air-laid nonwoven fabric having a basis weight of 58 g/m² andcomposed of only pulp was prepared, and impregnated with an emulsion ofa polyacrylic ester resin to a solid content of 8 g/m², followed bydrying at a hot air temperature of 130° C. to form a resin type air-laidnonwoven fabric.

The physical properties of the nonwoven fabrics obtained in Examples andComparative Examples described above are shown in Table 1. TABLE 1Comparative Comparative Comparative Example 1 Example 2 Example 3Example 1 Example 2 Example 3 Front and Composition PET/PE ConjugateFiber 100%  90% 100%  100%  100%  Acrylic resin Back (2.2 dt × 5 mm) 8g/m² Surface Pulp — 10% — — — Pulp Layers Basis Weight g/m² 6 8 10 4 1655 g/m² Internal Composition PP/PE Conjugate Fiber 30% 25% 40%  8% 70%Layer (1.7 dt × 5 mm) Pulp 70% 75% 60% 92% 30% Basis Weiqht g/m² 33 49100 52 32 Physical Basis Weiqht (g/m²) 45 65 120 60 55 63 PropertiesThickness (mm) 0.54 0.68 1.30 0.78 0.50 0.73 of Nonwoven Dry StrengthLengthwise 11.3 9.3 34.5 5.1 15.0 7.9 Fabric (N/25 mm) Crosswise 9.9 9.033.8 4.7 15.2 6.0 Ratio 0.88 0.97 0.98 0.92 1.01 0.76 (Crosswise/Lengthwise) Wet Strength Lengthwise 9.0 7.9 31.2 2.3 14.8 3.1 (N/25 mm)Crosswise 9.2 7.5 30.8 19 15.0 2.5 Ratio 1.02 0.95 0.99 0.83 1.01 0.81(Crosswise/ Lengthwise) Dry-Wet Wet/Dry 0.80 0.85 0.90 0.45 0.99 0.39Strength Ratio (Lengthwise) Water g/m² 538 790 1080 960 385 882Absorption g/g 12.0 12.2 9.0 16 7 14

INDUSTRIAL APPLICABILITY

The dry-process nonwoven pulp fabric composed of united layer structuresof the present invention is suitable for wipers for material use such asfloor use, kitchen use or automobile use, for kitchen sheets forcleaning of cooking utensils, wiping of foodstuffs, drip absorption andthe like, or for wipers for human use such as face use, hand use or bodyuse. Further, the nonwoven fabric is similarly suitable for hygienicmaterials such as sanitary goods and diapers, or for medical use.

1. A dry-process nonwoven pulp fabric composed of united layerstructures, which contains heat-bondable synthetic fibers and comprisessurface layer portions on both sides in which the synthetic fibers areheat bonded to one another and which has a basis weight (METSUKE) ofmore than 5 g/m² to 12 g/m², and an internal layer portion in whichheat-bondable synthetic fibers and pulp fibers are mixed at a ratio of20/80 to 60/40% by weight and the synthetic fibers and/or the syntheticfibers and pulp fibers are heat bonded to one another and which has abasis weight of 8 to 240 g/m², wherein the front and back surface layersand the internal layer portion are united as a whole by heat bonding ofthe synthetic fibers to one another, the ratio of a strength in alengthwise direction to that in a crosswise direction is from 0.8 to 1.2in both dry and wet states, the ratio of a strength in a dry state tothat in a wet state is from 0.6 to 1.1, the water absorption is from 8to 20 g/g, and the total basis weight is from 20 to 250 g/m².